The invention relates to the preparation of phthalic anhydride by gas phase oxidation of o-xylene. To this end, a gas stream which comprises molecular oxygen and o-xylene is generally passed through a multitude of tubes disposed in a reactor, in which a bed of at least one catalyst is present. To regulate the temperature, the tubes are surrounded by a heat carrier medium, for example a salt melt.
In spite of this thermostating, so-called “hotspots” form in the catalyst bed, at which the temperature is higher than in the remainder of the catalyst bed. These “hotspots” cause side reactions, such as the total combustion of the starting material, or lead to the formation of undesirable by-products which can be removed from the reaction product only with a high level of complexity, if at all, for example phthalide or benzoic acid.
To attenuate these hotspots, there has been a move in industry to arranging catalysts of different activity layer by layer in the catalyst bed, in which case the less active catalyst is generally arranged in the fixed bed such that the reaction gas mixture comes into contact with it first, i.e. it is present in the bed toward the gas inlet, whereas the more active catalyst is present toward the gas outlet from the catalyst bed (DE-A 25 46 268, EP 286 448, DE 29 48 163, EP 163 231).
To bring the reactor into operation, or to “start it up”, the catalyst bed is typically brought by external heating to a temperature which is above the later operating temperature. As soon as the oxidation reaction commences, the reaction temperature is maintained by the marked exothermicity of the reaction and the external heating is reduced and finally switched off. However, the formation of a marked hotspot prevents a rapid startup phase, since, from a particular hotspot temperature, the catalyst can be damaged irreversibly. The loading of the gas stream with the hydrocarbon to be oxidized is therefore increased in small steps and has to be controlled very carefully.
WO 98/00778 discloses that the addition of temporary activity attenuators can lead to a shortening of the startup phase.
In spite of the proposed improvements mentioned above, long startup times of from 2 to 8 weeks or longer have been required to date. “Startup time” describes the time which is needed to bring the feed of the hydrocarbon to the desired end loading, i.e. to bring the oxidation to the steady state, without irreversibly damaging the catalyst. In this context, it should be ensured in particular that the hotspot does not exceed a certain critical value, since the selectivity and the lifetime of the catalyst are otherwise significantly impaired.
On the other hand, the salt bath temperature on startup cannot be selected at as low a level as desired, since increased contents of unconverted hydrocarbon and underoxidation products otherwise occur in the reaction product, which can lead to exceedance of emission and/or quality requirements.
In the case of the industrially important oxidation of o-xylene to phthalic anhydride, the end loading is, for example, 80 g of o-xylene/m3 (STP) of air or more. The catalysts based on vanadium oxide and titanium dioxide used to date are started up at temperatures of from 360 to 400° C. This ensures that the residual amount of o-xylene and the content of the phthalide underoxidation product are within the emission and quality requirements. In the course of the formation phase which then follows, the salt bath temperature is lowered (to typically about 350° C.) and the loading can be increased in parallel to target load.
WO 2005/063673 describes a process for preparing unsaturated aldehydes and/or carboxylic acids by partial oxidation over a fixed catalyst bed, wherein the reactor comprises a reaction zone in which unsaturated aldehydes are obtained as the main product, and a layer of inactive material is inserted within this reaction zone at the point at which the position of the hotspot is expected.